A PBGF (photonic band gap fiber) is an optical fiber including a clad section in which a refractive-index distribution in a cross section perpendicular to a light waveguide direction exhibits two-dimensional periodicity. The periodic structure of the refractive-index distribution forms photonic band gaps. The formation of the photonic band gaps realizes a selective wavelength characteristic, thereby keeping light with a specific wavelength corresponding to the periodic structure within a core section, or, on the contrary, removing light with a specific wavelength from the core section. Use of this characteristic of the PBGF realizes an optical filter (optical fiber-type optical filter).
Conventional PBGFs are mainly divided into two types, namely, holey fiber types and solid types.
In a holey fiber PBGF, the periodic structure is realized by arranging a plurality of holes. In a solid PBGF, on the other hand, the periodic structure is realized by arranging a plurality of high refractive-index sections. The high refractive-index sections are portions having a refractive index higher than that of quartz used as a base material of the optical fiber. Particularly, in a case of a solid PBGF in which a core section has a high refractive index and a waveguide structure, an optical filter can be produced with a function of removing, from the core section, light with a specific wavelength corresponding to the periodic structure.
Patent Literatures 1 to 6 disclose techniques related to these conventional PBGFs. Particularly, Patent Literature 6 discloses a technique of using, as a fiber for amplification of light, an optical fiber-type optical filter employing a conventional PBGF.